I don't know for a fact, but it might not be possible for your phone to receive MMS from AT&T phones. It is only because that prior to the merger, AT&T and Cingular picture messaging were two seperate formats. Not saying it isn't possible to send and rcv MMS between AT&T and Cingular.
Have LG L1400 camera phone. Cant receive picture message from some phones. In particular samsung E316 and audiovox smart phone. Both with AT&T. I'm with cingular.I get tone that alerts of a multimedia message and when I go to inbox to retrieve it says one or more of components were deleted by mms adaption. Either message was too large or components were unsuitable for your terminal.I keep image size set at largest. have go picture from motorola phones so far. cingular has tried various things. nothing helps only get message, but no photo.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic